Evaluation of the CO_2 Sequestration Capacity in Alberta's Oil and Gas Reservoirs at Depletion and the Effect of Underlying Aquifers

摘要

Geological sequestration of CO_2 is an immediately available means of reducing CO_2 emissions into the atmosphere from major point sources, such as thermal power plants and the petrochemical industry, and is particularly suited to landlocked Alberta. Trapping CO_2 in depleted hydrocarbon reservoirs and through enhanced oil recovery (EOR) will likely be implemented first because the geological conditions are already well known and the infrastructure is partially in place. Assuming that the volume occupied by the produced oil and gas can be backfilled with CO_2, the ultimate theoretical CO_2 sequestration capacity in Alberta's gas reservoirs not associated with oil pools is estimated to be 11.35 Gt. The sequestration capacity in the gas cap of oil reservoirs is 865 Mt of CO_2, but this additional capacity will become available sometime in the more distant future after both the oil and gas have been produced from these reservoirs. The theoretical ultimate sequestration capacity at depletion in oil pools in single drive and primary production is only 615 Mt of CO_2. Depending on the strength of the underlying aquifer, water invasion has the effect of reducing the theoretical CO_2 sequestration capacity of depleted reservoirs by 60% on average for oil pools and 28% on average for gas pools, if the reservoir is only allowed to be repressurized back to its initial pressure. Weak aquifers have no effect on reservoir CO_2 sequestration capacity. If other factors are taken into account, such as reservoir heterogeneity and CO_2 mobility and buoyancy, then the effective ultimate CO_2 sequestration capacity at depletion in hydrocarbon reservoirs in Alberta is estimated to be 9,860 Mt for non-associated gas pools and 242 Mt for oil reservoirs currently in single drive and primary production. However, most reservoirs have a relatively small CO_2 sequestration capacity, rendering them largely uneconomic. In addition, shallow reservoirs are inefficient because of low CO_2 density, while very deep reservoirs may be too costly because of the high cost of CO_2 compression, and also inefficient in terms of the net CO_2 sequestered. If only the largest reservoirs in the depth range of approximately 900 m to 3,500 m are considered, each with an individual capacity greater than 1 Mt CO_2, then the number of reservoirs in Alberta suitable for CO_2 sequestration in the short-to-medium term drops to 565 non-associated gas reservoirs and 22 oil reservoirs in single drive or primary production, with a practical CO_2 sequestration capacity of 2,660 and 115 Mt of CO_2, respectively. This practical capacity of Alberta's oil and gas reservoirs for CO_2 sequestration may provide a sink for CO_2 captured from major point sources that is estimated to last for a few decades.